1. Field of the invention
The present invention relates to a liquid crystal projection system and, more particularly, to a liquid crystal projection system making use of three reflective-type liquid crystal panels to accomplish equal optical paths during projection.
2. Description of Related Art
Conventional CRT displays are not suitable for large-size displays because of volume and cost considerations. The liquid crystal projection technique provides a compact display with quality equal to that of the conventional CRT display.
Projector architecture is either single panel or three panel. The single panel projector has low resolution and low brightness, but is cheap. The three panel projector has high resolution and high brightness, but is expensive. The projection technique used presently is almost the mature transmission-type. The transmission-type projector, however, has the problem of a low aperture ratio.
TW. Pat. No. TW556,041 discloses a reflective-type projection technique, which makes use of three reflective-type liquid crystal panels for image projection to provide a high aperture ratio and a high resolution, bright display.
FIG. 1 shows the architecture of the liquid crystal projection system disclosed in TW. Pat. No. TW556,041. There are optical paths from a light source 11 to three reflective-type liquid crystal panels 20, 21, and 22 and optical paths from the reflective-type liquid crystal panels 20, 21, and 22 to a projection lens 23. In the optical paths from the light source 11 to the reflective-type liquid crystal panels 20, 21, and 22, light from the light source 11 is split into two light beams by a dichroic mirror 14, one being dual primary color polarized light beams Rp and Bp, the other being a single primary color polarized light beam Gp. The dual primary color polarized light beams Rp and Bp are transmitted through a polarizing beam splitter 16 and a dichroic prism 18 to the reflective-type liquid crystal panels 21 and 22, respectively. The single primary color polarized light beam Gp is transmitted through a reflecting mirror 15 and a polarizing beam splitter to the reflective-type liquid crystal panel 20.
In the above description, the single primary color polarized beam Gp is transmitted to the reflecting mirror 15 with air as the medium and then to reflective-type liquid crystal panel 20 through the polarizing beam splitter 17 made of glass material. The dual primary color polarized light beams Rp and Bp, however, are transmitted to the reflective-type liquid crystal panels 21 and 22 through the polarizing beam splitter 16 made of glass material. Therefore, the optical paths of the dual primary color polarized light beams Rp and Bp and the single primary color polarized light beam Gp from the light source 11 to the reflective-type liquid crystal panels 20, 21 and 22 are unequal, resulting in distortion and reduction of uniformity of the light source of the liquid crystal projection system during projection.